1. Field of the Invention
The present invention relates to an ink-jet apparatus. More specifically, the invention relates to an ink-jet apparatus employing an ink-jet head having a plurality of ink ejection heaters in an ink path corresponding to each ejection opening.
2. Description of the Related Art
An ink-jet apparatus has been mainly known as a printing apparatus in printers, copy machines and so forth. Among various ink-jet apparatuses, an ink-jet printing apparatus of the type utilizing thermal energy as an energy for ejecting an ink and ejecting ink by bubble utilizing the thermal energy has been spread, recently. In addition, as other applications of this type of ink-jet printing apparatus, an ink-jet textile printing apparatus for performing printing of a given pattern, picture or synthesized image and so forth on a cloth is becoming known, in the recent years.
An ink-jet head to be employed in the ink-jet printing apparatus such as those set forth above, has an electrothermal transducing element (hereinafter also referred to as “heater”) as a source of the thermal energy. In most cases, the ink-jet head is provided with one heater corresponding to one ejection opening. On the other hand, there has been known the ink-jet head employing a plurality of heaters for each ink ejection opening, in a viewpoint discussed below.
Firstly, it has been known to drive a plurality of heaters alternately or selectively for the purpose of expanding life of the ink-jet head. Secondly, a plurality of heaters are employed for widening range of variation of ink ejection amount. In the second case, by selecting the heater to be driven and/or by selecting number of heaters to be driven, the ink ejection amount is varied.
In the later case, as more concrete structure, a plurality of heaters are arranged in alignment along an ink ejecting direction in an ink path communicated with the ejection opening of the ink-jet head so that a distance between the ejection opening and the driven heater is varied by selecting the heater to be driven (namely heater to be heated) and/or by selecting number of heaters to be driven. By this, the ejection amount of the ink can be varied.
On the other hand, as other structure, there has been known the ink-jet head, in which a plurality of heaters having mutually different surface areas are arranged in the ink path to make the ink ejection amount variable by varying the heater to be driven and/or by varying number of heaters to be driven.
However, when printing is performed employing the ink-jet head having a plurality of heaters corresponding to each of the ejection openings, there should arise the following problems.
The first problem occurs in so-called preliminary ejection to be performed as a part of an ejection recovery process.
More specifically, the preliminary ejection is to perform ink ejection from the ink-jet head irrespective of printing generally at the predetermined position in the printing apparatus. By this, the ink of increased viscosity in the ink-jet head is removed to maintain good ink ejecting condition. Such preliminary ejection is generally performed upon on-set of the power supply or at a given constant time interval during printing. However, in the case where ink ejection can be done at various ejection amounts by a plurality of heaters as set forth above, it is possible that printing is performed with setting the ink ejection amount to a small ejection amount. In such printing operation, when the preliminary ejection is performed in the small ink ejection amount, the effect of the preliminary ejection can be varied depending upon the ejection amount. For instance, amount of the ink of the increased viscosity and bubble to be discharged out of the inkjet head can become small in the case of small ink ejecting amount during the preliminary ejection. Also, it can be said that since the ejection amount and ejection speed in such mode of printing operation is small, viscosity of the ink is easily increased. Therefore, shortening the interval of the preliminary ejection may be required to lower a throughput in printing.
The second problem is related to stabilization of ink ejection amount.
In the ink-jet head of the type ejecting the ink employing the heater, when a head temperature or an ink temperature is varied, the ink ejection amount can be varied though the variation range is not significant, in general. Therefore, when the heat temperature is elevated according to progress of printing operation, a problem of variation of the image quality can be caused due to variation of the ink ejection amount. It has been previously proposed to provide a structure for stabilizing the ink ejection amount regardless of variation of the head temperature as disclosed in Japanese Patent Application Laid-open No. 31905/1993. Here, two sequential pulses are applied to the heater for one time of ink ejection for controlling the head temperature by controlling a pulse width or so forth (hereinafter, occasionally referred to as “pre-heat control”) of a preceding pulse among two pulses, so that a variation of the ink ejection amount can be decreased.
Incidentally, in structure to vary the ink ejection amount in a plurality of steps by selecting heaters to be driven in the ink-jet head by employing a plurality of heaters for ejection set forth above, it is of course desirable to maintain ejection amount stable at respective settings.
Japanese Patent Application Laid-open No. 132259/1980 discloses multi-tone expression in structure employing a plurality of heaters. However, it is clear that stabilization of the ink ejecting amount cannot be realized.
The third problem is a problem in the case where pre-heating control is employed relating to stabilization of the ejection amount associated with the second problem.
For stabilization of ejection of the inkjet head having a plurality of heaters, it is considered to employ the structure of the pre-heat control. However, there are little problems to be considered when optimal ejection amount is to be controlled at respective ink ejection amount settings, such as a relationship between the drive heater in the set ejection amount and the heater performing preheating, a relationship between the set ejecting amount and the pulse width of the pre-heat pulse and so forth.
A fourth problem relates to multi-tone printing when a plurality of heaters are employed.
Regarding a plurality of heaters, the abovementioned prior art only shows structure for making the ink ejection amount variable by selectively driving a plurality of heaters. Therefore, it is possible that good quality of image cannot be printed even when it is applied for the multi-tone printing as is.
For example, when the ejection amount is varied in a relatively wide range by employing a plurality of heaters, the ejection speed for each ejection amount is significantly varied associating therewith. In this case, so-called serial type printing apparatus, in which printing is performed with scanning the inkjet head, a depositing position of an ejected ink can be offset by variation of the ejecting speed. As a result, a problem is encountered by lowering of the image quality.